Discovery of pulsation dropout and turn-on during the high state of the accreting X-ray pulsar LMC X-4

TL;DR

This study reports the discovery of pulsation dropout and turn-on phenomena in LMC X-4 during high luminosity flares, revealing dynamic changes in pulsation behavior linked to accretion states and emission geometry.

Contribution

It provides the first detailed timing analysis of pulsation behavior during super-Eddington flares in LMC X-4, highlighting rapid pulse turn-on and phase shifts.

Findings

Pulsations occur only during specific periods around flares.

Pulse profiles show a phase shift at flare onset.

Pulsations are weak or absent outside flare periods.

Abstract

Two \textit{NuSTAR} observations of the luminous X-ray pulsar LMC X-4 in October and November 2015 captured several bright accretion flares from this source, which has a long history of stable pulse and superorbital behavior. We present a timing analysis of these data in which we detect a rapid pulse turn-on in association with the accretion flares, during which the source reaches super-Eddington luminosities. Pulsations, which are normally seen from this source, are found to only occur for approximately one hour before and during the bright flares. Beyond one hour before and after the flares, we find pulsations to be weak or nonexistent, with fractional RMS amplitudes of less than 0.05. At the onset of the flare, the pulse profiles exhibit a phase shift of 0.25 cycles that could be associated with a change in the emission geometry. This increase in pulse strength occurring well before the flare cannot be explained by the propeller effect, and potentially offers a connection between the magnetic properties of pulsars that accrete close to their Eddington limits and ULX pulsars.